The invention relates to a method for separating a plurality of at least partly superimposed, flat articles at a predetermined point. The flat articles are, for example, made from paper, cardboard, fabric, felt or plastic. The plurality of flat articles to be separated may be, for example, a multipage printed product made from paper, plastic or cardboard, which is to be opened at a predetermined point. The articles may also be a stack or scale formation of printed products or of flat articles made from cardboard, fabric, felt or plastic and from which stack or scale formation a specific number of articles are to be removed by separation of the plurality at a predetermined point.
Superimposed, flat articles are frequently separated from one another by inserting a separating element, e.g. a separating wedge between the articles to be separated. The more congruent, thin and sensitive to mechanical damage the articles in question are, the more difficult it is to successfully effect separation using a separating element. On the one hand, it is difficult to aim the element precisely at a predetermined point and, on the other hand, the risk of the separating element damaging the narrow sides or edges of the flat articles is not negligible.
Therefore, superimposed, flat articles are advantageously displaced against one another parallel to their principal surfaces, prior to an effective separation. By such displacement, areas of the principal surfaces of individual articles are exposed. These principal surface areas are much better suited to being acted upon by a mechanical separating element than the narrow side areas of the articles.
CH-440339 describes a method for displacing, relative to each other, two directly superimposed articles within a plurality of superimposed articles. The CH ""339 method employs a displacement force having a component parallel to the principal surfaces of the articles. In this method, there is no action on individual narrow sides or edges of the articles. To enable desired displacement between two specific articles of the plurality of superimposed articles (predetermined separation point), while the other articles remain substantially undisplaced with respect to one another, the adhesion between the two specific articles is reduced, compared to the adhesion between the other articles, by measures suitable for reducing the friction coefficient between the two specific articles.
Measures for reducing the friction coefficient include treating at least one of the surfaces of the articles to be displaced with a lubricant or smoothing the surface or positioning a sliding film between the articles to be displaced. Examples of lubricants are waxes, high-viscosity silicone oils or dry lubricants.
Building upon the teaching of the aforementioned patent, CH-534588 describes how a stack of flexible sheets is made separable at a plurality of different, predetermined points, by applying friction coefficient reducing measures to different, predetermined sheets in non-overlapping areas. The stack is separated at a specific one of the predetermined separation points by directing the displacement force to the associated or corresponding treated area.
It has been found that the processes according to CH-440339 and CH-534588 give satisfactory results in many applications. However, it has also been found that in other cases the reliability with which the displacement is effected exactly at the predetermined point is inadequate. Accordingly, the prior art methods cannot be used when high accuracy is required. Such cases include the separation of pages of multipage printed products for the purpose of opening the product at a predetermined point. This is especially so when the individual pages of the printed products are made from a very smooth or correspondingly coated material and therefore adhere very little to one another, and/or when the individual pages of the printed products are made from different materials with different friction characteristics.
An object of the invention is to provide a method for separating a plurality of at least partly superimposed, flat articles at a predetermined point. With the aid of the method according to the invention it is to be possible to solve separating problems in applications wherein known methods are unusable, and/or to attain better results than possible with the known methods.
The method according to the present invention utilizes the fact that the adhesion between superimposed, flat articles can be increased by static charging of their surfaces. According to the inventive method, a static charge is produced on the surfaces of the superimposed, flat articles by passing the articles through an electric d.c. voltage field oriented substantially perpendicular to the principal surfaces of the articles. Due to the effect of the field, dipoles are oriented or charges are displaced perpendicular to the principal surfaces and positive or negative charges are concentrated on the two surfaces of each of the articles. These charges cause electric attraction forces between the surfaces of superimposed articles and, therefore, increase the mutual adhesion.
The effect of static charging of surfaces of superimposed, flat articles is reduced or prevented for at least one of two surfaces disposed at a predetermined separation point, by treating the surface beforehand with an anti-static agent. Due to the limited conductivity of the anti-static agent, charges cannot concentrate on the treated surface or they are very rapidly compensated. Thus, passage through the voltage field causes minimal or zero adhesion increase at the separation point, and causes much less adhesion increase at the separation point than at the non-treated surfaces. Therefore, the treated surfaces can be displaced or moved relative to each other with a lower force than is necessary to displace non-treated surfaces. Accordingly, low force displacement distinguishes the separation point or treated surfaces from the non-treated surfaces.
Application of a static charge for increasing the adhesion between flat articles not to be separated combined with a planned application of anti-static agent for suppressing the static-charge adhesion effect at a separation point can also be combined with a reduction of the friction between the flat articles to be separated by applying a lubricant, as described hereinbefore as prior art. Since for such a combination both the anti-static agent and the lubricant are advantageously used on the same surface areas, the two agents can be applied and used simultaneously in the form of a mixture in a single method step. It is also possible to add a luminophore to the anti-static agent or to the mixture of anti-static agent and lubricant. The luminophore is useful for checking the separation at the predetermined separation point with the aid of luminescence sensors.